Reverse transcriptase and genomic RNA are not associated catalytically in the core of RNA tumor viruses. Studies of DNA polymerase and virion structural proteins of MuLV nonconditional mutants indicate that reverse transcriptase is not the molecule in a leukemia virus which is inactivated by the Fv-1 restriction system, a naturally occurring cellular system which specifically inhibits the replication of certain murine leukemia viruses. It has been demonstrated that noninfectious MuLV virions lacking reverse transcriptase can abrogate this cellular restriction. Such noninfectious virions should be useful in studying the biochemical mechanism of this abrogation. BIBLIOGRAPHIC REFERENCES: Burstein, S. R., Gerwin, B. I., Taylor, H. and Westley, J.: Formal catalytic mechanism of ascorbate oxidase. Proc. of the First Int. Symposium on Iron and Copper Proteins. Advances in Experimental Biology and Medicine 74: 472-488, 1976. Duran-Troise, G., Bassin, R. H., Rein, A. and Gerwin, B. I.: Loss of Fv-1 restriction in BALB/3T3 cells following infection with a single N-tropic murine leukemia virus particle. Cell 10: 479-488, 1977.